The present invention relates to a liquid ejection apparatus for ejecting liquid as liquid drops from nozzle orifices and a method of controlling such an apparatus. More particularly, the invention relates to a liquid ejection apparatus capable of preventing the viscosity of the liquid from increasing by finely vibrating the meniscus of liquid in the nozzle orifice.
An image recording apparatus such as a printer, plotter, or the like is available as one kind of liquid ejection apparatus capable of ejecting liquid in the state of liquid drops. In this image recording apparatus, liquid ink is ejected from an ejection head and made to land on a recording medium such as paper. Thus, characters and images are recorded. Furthermore, recently, by making use of the characteristics permitting an extremely small amount of liquid to land on the medium accurately, application to various apparatus has been discussed. For example, display fabrication equipment for fabricating color filters such as for liquid crystal displays, electrode fabrication equipment for forming electrodes such as organic electroluminescent displays and FEDs (field emission displays), and chip fabrication equipment for fabricating biochips (biochemical devices) have been proposed.
In this kind of liquid ejection apparatus, the ejected liquid is exposed at the nozzle orifices and forms a meniscus (free surface of liquid). Through this meniscus, evaporation of the solvent component occurs, and the viscosity of the liquid may increase in the vicinities of the nozzle orifices. In an attempt to prevent this viscosity increase, the meniscus is vibrated finely to such an extent that liquid drops are not be ejected and the liquid is stirred. Such a vibrating operation is performed during the period in which liquid drops can be ejected. For example, as is disclosed in Japanese Patent Publication No. 10-81013A, a vibrating pulse is contained in a signal for driving a pressure generating element, and the vibrating operation is performed by selectively supplying the vibrating pulse to the pressure generating element.
In this case, a drive signal generator generates a series of drive signal in response to reception of a trigger signal. For example, as shown in FIG. 10, in an ink jet printer that is one kind of liquid ejection apparatus, a series of drive signal COM is generated over a signal generation period T when a trigger signal PTS is received, in order to accurately define the landing positions of liquid drops. Specifically, the aforementioned trigger signal is created by multiplying (increasing the frequency several-fold) the output from a linear encoder indicative of the carriage position. As a consequence, the deviation between the scan position of the recording head and the signal generation timing can be reduced as less as possible. The accuracy of the landing positions can be accordingly enhanced. Furthermore, in the above printer, a trigger signal PTS is generated every time when the printing for a 1-dot area (pixel) is performed.
Incidentally, this kind of liquid ejection apparatus is required of high-frequency ejection of liquid drops since the processing speed can be increased and the landing density can be improved. For example, in an ink jet printer, if the ejection frequency of ink drops can be increased, the scanning speed of the recording head can be increased accordingly, while maintaining the image resolution (quality). In other words, the image resolution can be enhanced while maintaining the scanning speed of the recording head unchanged.
In order to attain high-frequency ejection of liquid drops, it is necessary to shorten the interval at which ejection pulses are generated regarding the ejection pulses for ejecting the liquid drops. However, the above-described vibrating pulse is used only to vibrate the meniscus and does not involve ejection of the liquid drops. Therefore, in the configuration where a vibrating pulse is contained within each ejection period, a time for the vibrating pulse is necessary. Accordingly, the ejection interval of the liquid drops is prolonged by an amount corresponding to the vibrating pulse. This creates an impediment to high-frequency ejection of liquid drops.
Furthermore, the degree of necessity of the vibrating operation varies according to the kind of the ejected liquid. That is, some liquids need frequent vibration in non-ejection periods, while others do not. Therefore, performing the operation impartially regardless of the kind of liquid is not efficient.